"The Investigative Division of the state Ethics Commission has initiated a full investigation in relation to the complaints that you filed," Ethics Commission Executive Director Robert P. Caruso wrote to Stilp in a July 17 letter.
Reached Monday, Caruso would not comment on the matter, saying the commission did not confirm or deny the existence of a pending investigation.
The Ethics Commission is an independent agency empowered to investigate alleged violations of the state Ethics Act, which applies to public officials and employees. It does not have prosecutorial powers and cannot bring criminal charges, but it can impose fines and refer matters to law enforcement authorities.
The commission's probe is the third launched since the sting became public. The House Committee on Ethics in late March began looking into whether the four legislators violated the chamber's ethics rules, an inquiry that remains open.
And Philadelphia District Attorney Seth Williams announced last month that he had assembled a team of prosecutors to present evidence from the sting case to an investigative grand jury. Williams pledged to complete his investigation in "months, not years."
The sting investigation began in 2010 under Attorney General Tom Corbett, now the governor, and caught four state legislators and a Philadelphia Traffic Court judge on tape accepting cash, money orders, or jewelry.
After taking office in January of last year, Kane shut down the inquiry. She has said she did so because she believed it was poorly conceived, ineptly managed, and possibly tainted by racial profiling.
All five elected officials captured on tape are African American. They are Reps. Ron Waters, Vanessa Brown, Michelle Brownlee, and Louise Bishop, and onetime Traffic Court Judge Thomasine Tynes.
Prosecutors who launched the investigation argued that Kane wrongly ended a probe that had caught officials accepting money and that had the potential to capture more.
None of the lawmakers could be reached Monday for comment.
Waters has said he might have accepted a birthday gift from the sting's undercover operative, Philadelphia lobbyist Tyron B. Ali. Brownlee has said she could not remember whether she took money. Bishop has denied accepting money from Ali or even knowing him, and Brown has denied wrongdoing.
In an interview Monday, Stilp said he hoped the Ethics Commission could move swiftly with the inquiry and seek "the maximum fines."
"I hope the commission is very, very stern with these legislators," said Stilp. "It's time they step up to the plate."
He also noted that when the state Ethics Commission finds criminal activity in cases, it normally refers them to the Attorney General's Office, possibly setting the stage for an awkward situation between the commission and Kane.
A spokesman for Kane declined to comment Monday.